Glass antennas in which conductive lines are formed on a vehicle window glass are superior to conventional rod antennas in that (i) they are designed not to protrude outward, (ii) there is little danger of breakage, and (iii) they do not cause wind noise. For these and other reasons, such glass antennas are in widespread use.
When receiving radio waves in the VHF band (FM band and TV band), the antenna element often receives reflecting waves as well as direct waves from the broadcasting antenna. The reflecting waves are reflected from the ground and from structures such as buildings. Sometimes, not only one reflecting wave but also several reflecting waves reach the antenna element from several reflection paths. When two radio waves having opposite phases are received, the received radio signal becomes weaker.
Thus, diversity antenna systems have been developed and put into practice, in which two antenna elements with different directionality are provided, and while the vehicle is in motion, the antenna element with the stronger reception signal is selected.
The antenna elements constituting such a diversity antenna system have to have different directionality. For example, in the automobile glass antennas disclosed in JP H10-13127A (1998) and JP H10-242730A (1998), antenna elements are provided on the left and right side windows to form a diversity antenna.
When antenna elements are provided on the left and right side windows, the antenna elements are provided at different locations, and the influence of the metal monocoque constituting the car body on each of the antenna elements is different. Therefore, the two antenna elements have different directionality, so that they preferably can be used for a diversity antenna.
In the vehicle glass antenna disclosed in JP H09-181514A (1997), two antenna elements are provided at the margin portion above the heating conductive lines on a rear window glass, and at least one antenna element is provided at the lower margin portion. These upper and lower antenna elements constitute a diversity antenna.
According to the “Embodiments of the Invention” of this publication, “With respect to the two horizontal antenna elements provided at the upper margin portion, in order to efficiently utilize the length from one lateral edge to the other lateral edge of the window, the horizontal length is ensured by partitioning them not vertically into two, but partitioning them horizontally into two.” Moreover, a complicated branching pattern is shown as the pattern of the two antenna elements provided at the upper margin portion. It is explained “there are one or two antenna elements provided at the margin portion below the heating lines, and if two antenna elements are provided, they should be divided into left and right parts.”
Furthermore, it is explained that “in diversity receiving, (i) audio signals of FM broadcasting waves should be diversity-received with one antenna element at the upper margin portion and one antenna element at the lower margin portion, and signals that are not audio signals of FM broadcasting waves, such as text signals, should be diversity-received with the other antenna element at the upper margin portion and the other antenna element at the lower margin portion or an antenna element provided at a separate location, or (ii) signals that are not audio signals of FM broadcasting waves should be diversity-received with one antenna element at the upper margin portion and one antenna element at the lower margin portion, and audio signals of FM text broadcasting waves should be diversity-received with the other antenna element at the upper margin portion and the other antenna element at the lower margin portion or an antenna provided at a separate location.”
The present applicant has disclosed a vehicle glass antenna system in WO 00/70708.
In the glass antenna system shown in FIG. 3 of that application, an FM (main) antenna of one conductive line is provided above a defogging heater, and an FM sub-antenna of one conductive line is provided below the defogging heater on a rear glass, thus constituting a diversity antenna. Furthermore, in this antenna system, an AM antenna is provided above the FM (main) antenna.
If diversity reception is performed with two antenna elements provided at the margin portion above and the margin portion below the heating lines of a vehicle rear window glass, as in the vehicle glass antenna system disclosed in JP H09-181514A (1997), the following problems occur.
The antenna elements are provided at different heights at the margin portion above and the margin portion below the heating lines, so that a difference in the basic receiving sensitivities of the two antenna elements occurs. More specifically, the receiving sensitivity of the antenna provided at the lower margin portion often deteriorates. Furthermore, it is also susceptible to the adverse influence of the rear tray of the car body. With the vehicle glass antenna disclosed in JP H09-181514 (1997), it is sometimes difficult to attain a consistently superior receiving sensitivity, even when performing diversity reception.